1. Field of the Invention
The present invention relates to nonionically, hydrophilically modified polyisocyanate compositions prepared from both aliphatic and aromatic diisocyanates, a method for their preparation, and an aqueous coating composition containing these polyisocyanate compositions.
2. Description of the Prior Art
The ecological compatibility of coatings and coating compositions is playing an increasingly important role in surface technology. One problem is the reduction of the quantities of organic solvents used in coatings and coating compositions. An important contribution to the resolution of this problem is being made by the aqueous two-component polyurethane coatings which have become known in recent times. These systems permit the production of high-quality coatings, exactly like the corresponding two-component, solvent borne polyurethane coatings.
Aqueous two-component polyurethane coatings are described, for example,in EP-A 0,358,979, EP-A 0,543,228, EP-A 542,105 and in the PCT Applications published under the publication numbers 9305087 and 9309157. The compositions of EP-A 0,358,979 are based on selected water-soluble or water-dispersible acrylic resins and preferably low molecular weight aliphatic polyisocyanates. The coatings of this prior publication dry relatively slowly, so that they are of only limited suitability for fields of application where rapid drying is critical, such as the varnishing of wood and furniture.
These comments also apply to the two-component systems of Japanese Patent Application 58-66736, published under unexamined application number 191770/1984. The coating compositions described in this prior publication, based on tests carried out by the inventors, show totally inadequate film-forming properties, i.e., mat films having severe surface defects are invariably obtained. In addition, drying times of three days are necessary even when aromatic polyisocyanates are used.
Based on these observations, it is an object of the present invention to provide new, aqueous two-component polyurethane coating compositions have an acceptable processing time, and yet cure at room temperature as rapidly as the known, solvent-containing two-component polyurethane coatings of the prior art.
This object may be achieved by using the polyisocyanate compositions according to the invention, described in more detail below, as the polyisocyanate component in aqueous two-component polyurethane coating compositions as set forth herein. The polyisocyanate compositions according to the invention contain both aliphatic and aromatic diisocyanates in chemically incorporated form. The aromatic diisocyanates are preferably part of the polyisocyanates which contain isocyanurate groups, while the aliphatic diisocyanates in particular can be part of the conventional lacquer polyisocyanates.
The preparation of water-dispersible polyisocyanates by the reaction of diisocyanates with monofunctional polyethylene oxide polyethers is described in a series of publications such as EP-A 0,013,112, EP-A 0,061,628, DE-OS 2,908,844, DE-OS 2,921,681, DE-OS 4,136,618, GB-PS 1,444,933, US-PS 3,996,154 and U.S. Pat. No. 5,200,489. HDI and TDI are among the suitable starting diisocyanates mentioned. However, none of the these publications disclose hydrophilically modified polyisocyanate compositions in which both HDI and TDI are present in chemically incorporated form. Consequently, the advantages of the polyisocyanates compositions according to the invention are not disclosed in these prior publications. These advantages include not only the favorable properties previously set forth but also a particularly marked, outstanding compatibility with the aqueous polyacrylic dispersions, which are preferably employed in aqueous two-component polyurethane coating compositions.